It usually means beef, and it can be any cut of beef, but rib or sirloin is most common. It’s then served as the main part of a “traditional” Sunday roast, consisting of roast beef, roast potatoes (plus, if you’re a member of my potato-obsessed family, a couple of other kinds of potato as well), Yorkshire puddings, gravy and a couple of veg. There’d normally be a sweet pudding afterwards - creme caramel or sherry trifle were favourites in my family, but those aren’t “traditional”.
Once upon a time, it was a way of getting the whole family together, on the one day no-one would be working, usually after church. The leftovers would then form the basis of the next couple of meals - Bubble and Squeak from the veggies, cottage pie from the meat, soup from the bones, etc.
Sunday roasts are more commonly eaten in pubs these days than cooked at home, though I cook them occasionally - normally when we’ve got friends round, as it’s too much for two. I cheat and buy ready-made frozen Yorkshire pudding, but I make cracking gravy and roast spuds.